2-Chloropropionaldehyde (hereinafter, is referred to as CPA) is a useful compound as a raw material for the organic synthesis of medicaments, agricultural chemicals, etc. However, since the compound is a very unstable compound, it can not be stably stored for a long period of time. Accordingly, in the case of using the compound, it must be prepared each time just prior to use and immediately supplied for a reaction, and hence the use of the compound is very troublesome.